1. Field of the Invention
This invention relates to a hydraulic shift controller for an automatic transmission of a vehicle, and more particularly to improvements in a hydraulic shift controller for an automatic transmission for a vehicle, wherein there are provided at least a first transmission section capable of automatically shifting gears in association with at least vehicle speed and engine load and a second transmission section capable of automatically shifting between the lower gear side and the higher gear side, and wherein the first transmission section and the second transmission section are simultaneously or alternately shifted, to thereby achieve multi-gear shifts.
2. Description of the Prior Art
Along with a rapid application of the automatic transmissions to motor vehicles, with mainly the intention of improved fuel consumption rate, there have been commonly adopted transmissions wherein a so-called overdrive device, in which a transmission gear ratio is less than 1, is connected in series to an output shaft of a first transmission section capable of automatically shifting gears in association with vehicle speed, throttle opening and the like as a second transmission section.
Furthermore, such a transmission is also know wherein, based on a function of the second transmission section capable of shifting between the lower gear side and the higher gear side as seen with the above-described overdrive device, the second transmission section is caused to be actively tuned to shifts of the first transmission section, and the first transmission section and the second transmission section are simultaneously or alternately shifted, whereby the shift control shown in FIG. 4 is performed for example, so that multi-gear shifting of six forward gears can be achieved.
The above-described arrangement makes it possible for the existing automatic transmission to be utilized as the first transmission section and changes in design are minimized for the manufacturing advantage, so that multi-gear shifts can be achieved. As a result, many advantages can be offered such that the rate of fuel consumption is improved, shift shocks are reduced and the load is relieved from a frictionally engaging device due to making the gear shifts into multi-gear shifts.
However, in the automatic transmission, wherein the first and the second transmission sections are shifted simultaneously or alternately to achieve the multi-gear shifts, there has existed a disadvantage in that, when an acting force of the frictionally engaging device for the high gear shift of the second transmission section is varied only in accordance with the throttle opening, as performed in an ordinary up shift, it has been difficult to obtain satisfactory shift characteristics (including the problem of durability) over all the shifts achieved by the high gear shift of the second transmission section.
More specifically, as apparent from FIG. 4, as for the shifts from 1st gear to 2nd gear, from 3rd gear to 4th gear and 5th gear to 6th gear, up shifting of the automatic transmission as a whole is realized only by a high gear shift of the second transmission section, with the first transmission section remaining as it is. In this case, the input torques to the second transmission section are greatly varied in accordance with each gear of the first transmission section, whereby a transmitted torque required from the frictionally engaging device in the second transmission section is greatly influenced by the gear of this first transmission section (See FIG. 6 for example). As a consequence, in the case where the hydraulic pressure of the frictionally engaging device for shifting the second transmission section into the high gear side remains the same at the time of the each each shift, when 1st gear is shifted into 2nd gear for example, the engaging time duration is prolonged due to the deficiency of the hydraulic pressure, whereby the durability is considerably lowered. Or, when 5th gear is shifted into 6th gear, a sudden change in the torque occurs due to the excessively high hydraulic pressure, whereby the shift shock is increased.